Typically, the elements within conventional optical communication systems are designed to perform only a single function. That is, optical source components are not disposed to be used interchangeably used as in-line amplifiers and optical sources, and vice-versa. Once an optical signal has been produced by an optical source, the signal is typically coupled into the fiber optic network using a conventional lens arrangement. Within the network, separate doped fiber or other semiconductor laser devices may then be used as in-line amplifiers to maintain a requisite level of optical signal power.
Considering a specific example, a conventional optical source component may include a semiconductor laser diode having an output facet optically coupled to a discrete optical power amplifier. The diode and amplifier may be formed together on a single crystal, and are maintained in optical alignment using various techniques. Unfortunately, the laser diode and light amplification elements are so configured so as to be capable of serving only as a source of optical energy. That is, the individual elements comprising the optical source are not designed to be decoupled in order to make possible the performance of a separate signal amplification function.
Research efforts have thus far been directed to improving the performance characteristics of the discrete optical source and amplification elements included within existing communication systems. Typically, efforts have concentrated upon enhancing emission wavelength stability, spectral purity, and modulation speed. Unfortunately, little progress appears to have been made in devising optical devices capable of performing more than a single function. It is believed that an optical device configured to perform, for example, both signal generation and amplification operations, would facilitate new network architectures for optical communication systems and would further lead to novel device applications.
Several obstacles have thus far precluded development of, for example, semiconductor laser devices capable of operating both as an amplifier and as a signal source. As is well known, when the reflectivity of both facets of a semiconductor diode laser are suppressed, a single-pass or "traveling-wave" amplifier (TWA) is formed. The required suppression of facet reflectivity is typically performed by applying an anti-reflection (AR) coating on each laser facet. However, the application of such an anti-reflection coating to the laser facets permanently renders the laser device incapable of operating as an optical source. Hence, semiconductor diode lasers have been configured to perform exclusively either signal generation or amplification functions.
Moreover, reducing facet reflectivity using AR coatings generally requires that precise control be maintained over both the refractive index and coating thickness. For example, control of coating thickness to within a few nanometers is generally required to achieve optimal performance. In addition, it has been determined that different ideal coating thicknesses exist for TE and TM mode operation of TWA devices. This finding has further complicated the problem of fabricating a TWA by using AR coating techniques to reduce facet reflectivity. Accordingly, it would be desirable to provide a semiconductor laser TWA not reliant upon an AR coating for facet reflectivity reduction.
Research efforts within the field of optical communications have also focused upon the development of diode laser transmitters for providing a modulated optical signal to a fiber optic communications network. Both the intensity and the emission wavelength of the diode laser may be modulated by varying the applied current. For example, digital coding of an optical input signal may be achieved by alternately turning the laser diode current on and off. However, such current modulation techniques may be unable to provide the modulation speed required for high capacity fiber optic networks. As a consequence, other techniques of high-speed optical modulation using external modulators have been investigated. Unfortunately, the size and temperature sensitivity of external modulators may render these devices inappropriate for certain fiber optic network applications. Accordingly, it would be desirable to provide a compact optical source capable of producing a high-speed modulated optical output signal.